You should simply go ahead and buy the interface, and everything will become clear in no time.
The playback row is audio “returns” from, software (Windows, Cubase, whatever other program uses the interface for sound output…)
For each Hardware output channel there is usually one software playback channel. There is no direct relation between playback channels and Cubase Audio channels. The totalmix playback channels are the outputs for Cubase. So every output bus in Cubase is routed to a playback channel which has to be routed to a physical interface output (Totalmix output channel) via totalmix. No Cubase (or other software programme audio output) can be routed to a totalmix output channel directly.